A power bank is a portable battery that stores electrical charge and transfers it to your phone, tablet, or laptop when you are away from an outlet

A power bank is essentially a rechargeable battery in a box. You charge it at home or at an outlet, then carry it with you. When your device runs low, you plug it into the power bank instead of hunting for a wall outlet. The power bank drains instead of your device, buying you hours of extra use.

Power banks range from small enough to fit in a pocket to larger units that charge multiple devices at once. They work with phones, tablets, laptops, wireless earbuds, smartwatches, and anything else that charges via USB or USB-C. Most power banks cost between $15 and $80 depending on capacity and features.

Key Takeaways

  • A power bank stores charge at home and transfers it to your device when you are away from outlets, extending your battery life by hours.
  • Capacity is measured in milliamp-hours (mAh) — a 20,000 mAh power bank typically charges a phone twice, but the actual number of charges depends on your phone's battery size.
  • Fast-charging power banks require both a compatible charger and a compatible cable to work at higher speeds; standard charging works with any cable.
  • A power bank loses charge over time even when not in use, so it works best when you charge it before trips rather than storing it for months.

How capacity works and what the numbers mean

Power bank capacity is measured in milliamp-hours (mAh) or watt-hours (Wh). A 20,000 mAh power bank is larger than a 10,000 mAh one, but the actual number of times it charges your phone depends on your phone's battery size. A newer iPhone with a 3,500 mAh battery might get two full charges from a 20,000 mAh power bank. An older phone with a 2,000 mAh battery might get four charges from the same power bank. A tablet with a 7,000 mAh battery might get only three charges.

Manufacturers often list how many "charges" a power bank provides, but this number assumes a specific phone size and does not account for energy loss during transfer. A realistic estimate is that you lose about 20 to 30 percent of the power bank's capacity to heat and conversion, so a 20,000 mAh power bank delivers roughly 14,000 to 16,000 mAh to your device. For most people, a 10,000 to 20,000 mAh power bank covers a full day away from outlets.

Charging speed and which cables matter

Power banks charge your device at different speeds depending on the power bank's output, the cable you use, and your device's charging capability. A standard power bank outputs 5 watts and charges slowly but reliably. A fast-charging power bank outputs 18 watts, 30 watts, or higher, but only if three things align: the power bank supports that wattage, the cable supports it, and your device accepts it.

If you plug a fast-charging power bank into an older phone with a standard USB cable, it charges at standard speed — the phone straightforward does not accept the faster power. If you use a cheap or damaged cable with a fast-charging power bank, it charges at standard speed. Fast-charging power banks are worth buying only if your phone supports fast charging and you plan to use it regularly. For occasional backup charging, a standard power bank works fine and costs less.

USB-C, Lightning, and which ports your power bank needs

Power banks have two types of ports: input ports (where you charge the power bank itself) and output ports (where you charge your devices). Most modern power banks have a USB-C input port so you can charge them with the same cable as your phone. Older power banks use micro-USB input, which is slower and less common on new devices.

Output ports vary. A power bank might have one USB-A port (the rectangular one), one USB-C port, or both. Some include a Lightning port for iPhones, though this is less common because most iPhones now support USB-C. If you own multiple devices with different charging ports, a power bank with both USB-A and USB-C output lets you charge two devices at once. If you only charge phones and they are all USB-C, a single USB-C output is enough.

Power banks lose charge over time, even sitting unused

A fully charged power bank loses roughly 2 to 3 percent of its charge per month when sitting in a drawer. After six months, a power bank that left your house at 100 percent might be down to 85 percent. After a year, it could be at 70 percent. This is normal and happens with all rechargeable batteries.

For this reason, power banks work best as tools you use regularly rather than emergency backups you store for months. If you travel frequently or spend long days away from outlets, charge your power bank before each trip. If you store one for emergencies, check it every few months and top it up. Keeping a power bank in a cool, dry place slows the discharge rate.

When a power bank makes sense and when it does not

A power bank is useful if you spend six or more hours away from outlets regularly, use your phone heavily during the day, or travel to places where outlets are scarce. It is also practical if your device's battery has degraded over time and no longer lasts a full day. A power bank costs far less than replacing a phone battery and works when ready.

A power bank is less necessary if you work in an office with outlets, drive a car with a USB port, or have access to charging throughout your day. If your device battery is healthy and lasts all day, a power bank is backup insurance rather than a daily need. Consider your actual routine before buying one — a power bank sitting at home unused is money wasted.

Frequently Asked Questions

Can I use a power bank to charge my laptop?

Only if the power bank outputs enough wattage and has USB-C output. Most laptops need 30 watts or more to charge at full speed. A 20,000 mAh power bank with 30-watt USB-C output can charge a laptop, but it will charge slowly and may not keep up if you are using the laptop while charging. Check your laptop's charging requirements before buying.

Is it bad to leave a power bank plugged in overnight?

Modern power banks have built-in protection that stops charging once they reach 100 percent, so leaving one plugged in overnight will not damage it. However, keeping a battery at 100 percent for extended periods does slightly reduce its lifespan over years of use. For maximum longevity, unplug it once it is fully charged, but occasional overnight charging is safe.

Why does my power bank charge my phone slower than a wall outlet?

Power banks output less wattage than wall chargers. A wall charger might output 20 watts, while a power bank outputs 5 to 18 watts depending on the model. The power bank is also converting its stored energy, which generates heat and reduces efficiency. Slower charging is the trade-off for portability.

Can I take a power bank on an airplane?

Yes, power banks are allowed in carry-on luggage on most airlines, but checked baggage rules vary. The TSA allows power banks under 100 watt-hours in carry-on bags. Check your airline's specific policy before packing, as some restrict the number you can bring or require them to be in carry-on only.